Heats of adsorption of linearly adsorbed CO species on Co2+ and Co sites of reduced Co/Al2O3 catalysts in relationship with the CO/H-2 reaction

IR spectra in transmission mode are used to measure the heats of adsorption at different coverages theta:E(theta), of two linearly adsorbed CO species formed on Co2+ and Co degrees sites (noted LCo2+ and L-Co degrees) of reduced x% Co/Al2O3 (wt%,x L-Co degrees C (IR band at 2060 cm(-1)) where L-Co degrees C denotes a linear CO species formed on Co degrees sites modified by the C deposition. The heat of adsorption of the L-Co degrees C species varies linearly with its coverage from E-LCo degrees C (1) = 93 kJ/mol to E-LCo degrees C (0) = 165 kJ/mol. In the presence of H-2 with a ratio H-2/CO = 10, the C deposition is strongly decreased and the L-Co degrees species dominates the surface of the cobalt particles. However, in these conditions its hydrogenation into CH4 disturbs its adsorption equilibrium in a large coverage range and only ELCo degrees (1) = 108 kJ/mol has been determined. For a ratio H-2/CO = 3, the carbon deposition cannot be prevented leading to the formation of the L-Co degrees C species. It is shown that the E-LCo degrees C(theta) values are not modified by the presence of adsorbed hydrogen. The heats of adsorption of the different adsorbed CO species on Co degrees sites are consistent with some literature data on DFT calculations and experimental values obtained on model cobalt surfaces (i.e. single crystals). In particular, they confirm DFT calculations which indicate that the C deposition on reduced cobalt particles decreases slightly the heats of adsorption of the linear CO species adsorbed on Co degrees sites. (C) 2012 Elsevier B.V. All rights reserved.